Six alternative pathways for B12-dependent methyl-transfer have been elucidated. Three mechanisms lead to the release of methyl-radicals, two mechanisms produce carbanions and one produces a carbonium ion. The reaction with platinum salts has led to some basic biochemistry which is pertinent to the mechanism of action of the platinum anti-tumor drugs. Reactions with single electron reductants have possibilities for determining the mechanisms for B12-dependent free radical rearrangement reactions. Our work on B12-dependent ribonucleotide reductase is pertinent to the regulation of DNA synthesis.